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];f                    272                PRACTICAL ORGANIC CHEMISTRY

couple, and pyridine all behave in this way. The action of
iodine has already been explained on p. 252. Iron and its salts
are supposed to act by alternately passing from the ferrous to
the ferric state, the ferric salt delivering up its halogen in the
nascent state,

2FeBr2 + Br2 = 2FeBr3.
FeBr3 = FeBr2 + Br.

The action of aluminium and its compounds is not fully under-
stood. Pyridine probably acts by the intermediate formation of
the perbromide, as explained.

Unless a large excess of the hydrocarbon is present, the
action of the halogen will effect the substitution of a second
atom of hyrogen. By increasing the proportion of halogen,
all the hydrogen may be ultimately replaced by chlorine or
bromine. The second halogen atom enters the ortho- and para-
1 t ( :                   positions, never the meta. Another kind of compound is

obtained if the halogen is allowed to act in presence of sun-
light. In the 'case of benzene, the additive compounds, benzene
hexachloride and hexabromide, are then formed. They are very
unstable compounds, and readily give off hydrochloric and
hydrobromic acid. If boiled with alcoholic potash they are de-
composed, forming trichloro- and tribromo-benzene,

' l i                                 C6H6C16 + sKOH = CBH3C13 + sKCl + 3lIX>.

If chlorine and bromine are allowed to act upon an aromatic
hydrocarbon like toluene, which has a side-chain, substitution
may occur in the nucleus or the side-chain, according to the
conditions. Generally speaking, in the cold and in presence of
a "halogen carrier," nuclear substitution occurs, but at a high
temperature the halogen passes into the side-chain (see Prep.
86, p. 94).

,                 .         The halogen derivatives of the aromatic hydrocarbons, like

those of the aliphatic series, are colourless liquids or soli els,
denser than water, and possessing an agreeable smell, unless
the side-chain is substituted. The latter substances can often
be distinguished by their irritating action on the eyes and mucous
membrane of the nose (see Prep. 86, p. 194).
I1 1                             The halogen in the aromatic nucleus is much more firmly

fixed than in the case of the aliphatic compounds, e.g. bromo-
benzene is quite unaffected by most of the reagents which .act